A positron emission tomography camera (PET) is an invaluable but extremely expensive ($2,500,000) cross-sectional imaging device for in-vivo quantitative measurement of physiological and biochemical organ functions in both research and clinical environments. Positron radiopharmaceutical tracers are injected into the body. Each tracer molecule then emits two gamma-rays (511 KeV in energy). These gamma-rays are then detected by rings of tiny detectors surrounding the patient to create the cross-sectional density mapping of the tracers.
In any imaging process, from everyday 35 mm photography to a PET camera, the quality of the image depends on both the optical resolving power of the instrument and the amount of light received (photon statistics). In photography, the limiting factor is usually the resolving power of the lens/film system but not the photon statistics because the number of photons contributing to each picture-element (pixel) is extremely large that the statistical uncertainty or noise is very low. In PET or most "gamma-ray counting" imaging devices, the photon statistics is also a limiting factor in their image quality. After all, if there is enough time to count the incoming photons one by one, there cannot be too many coming in per unit time.
Recent efforts in improving the image quality of PETs are focused at improving the resolving power (intrinsic spatial resolution of the detectors) by packing smaller individual detectors into the detector rings (analogous to an insect with smaller size element in its compound eyes). However, this advance has not been accompanied by the required corresponding increase in the collected gamma-ray counts. As a result the high resolution images are noisier. Some image smoothing by pixel averaging is necessary to lower the noise in the image. But the pixel averaging procedure will lower the image resolution. Typically a 4-6 mm "intrinsic" resolution PET operates at a "usable" resolution of 6-8 mm. The difference between "intrinsic" resolution and "usable" resolution is particularly large for the very short-lived isotopes such as oxygen-15 (half-life +2 minutes) and rubidium-82 (half-life=1 minute) which decay away before enough counts are collected.
The conventional ways of increasing collected gamma-ray counts are:
(1) increasing the camera sensitivity--present cameras already have sensitivity close to 90% of the theoretical limit. Another 10% gain is very costly and hardly worthwhile. Furthermore, what is needed is a 100%-300% increase. PA1 (2) increasing the injected radiation dose--PET cameras are also plagued by a noise called "accidental coincidences". This noise increases drastically with increased radiation level as shown in FIG. 1. All cameras can easily reach the cross-over point where the "accidentals" starts overwhelming the "true" counts. Hence, most current cameras are designed to handle a maximum count rate at which the cross-over point occurs. Further increase in the counting speed is generally a waste. Many imaging procedures already inject radiation near the cross-over point, in particular, those tracers with very short half-lives. Hence this is not a viable solution. PA1 (3) increasing the imaging time--Again present imaging procedures are already stretching the data collection time to the practical limit of the half-life of the isotopes or other physiological and clinical requirements. Some procedures already require an hour or more of total imaging time. Hence this is also not a good solution. PA1 (4) multiple injections--It prolongs imaging time, decreases patient throughout, and it is also more vulnerable to patient movements, physiological changes between repeated studies. Isotope recirculations from prior injection may also be a problem.
One way of improving image quality by throttling the accepted counts has been disclosed in U.S. patent application Ser. No. 06/876,066 filed June 19, 1986, now U.S. Pat. No. 4,755,679 entitled "Method and Apparatus for Maximizing Counts of a PET Camera" which uses a dynamically adjusted energy acceptance window.
The present invention is directed to the use of a modulator to modulate the incoming counts and to keep the counting rate near the maximum camera limit.